Disclosed is a nucleic acid comprising: -a sequence encoding a toxic poly-peptide and -an intron that interrupts the sequence, whereby the intron is spliced in mammalian cells but not in insect cells to form an mRNA that is translated to form cell-toxic levels of the toxic polypeptide in mammalian cells but not in insect cells. Also disclosed is the use of the above-described nucleic acid in the manufacture of a medicament for treatment of a cancer.
